1. Field of the Invention
The invention pertains to the field of optical display systems. More particularly, the invention pertains to apparatus and methods for enhancement of a real image projection system through the use of one or more aspheric mirrors or corrective aspheric optical curvatures.
2. Description of Related Art
The invention pertains to a real image projection system, and in particular, to a system in which an image of a real object is formed in space, giving the illusion that a real object exists at that point in space, when in reality it does not. A variation of this type of system has existed for many years in the form of various toys and magic tricks. Most are in the form of dual facing parabolic mirrors of equal focal lengths, known as 360 (i.e., 360xc2x0) displays, which create the illusion that a real object exists at the vertex of the upper curved mirror, but in which the real target object is actually located within the device itself, at the vertex of the lower curved mirror. Thus, the device creates the illusion of an object floating above the unit, when actually the object is positioned within the device at a different location.
U.S. Pat. No. 5,886,818, to Summer et al. (1999), the complete disclosure of which is hereby incorporated herein by reference, discloses a real image projection system having some features in common with the present invention.
U.S. Patent No. 3,647,284, to Elings (1972), the complete disclosure of which is hereby incorporated herein by reference, referred to hereinafter as the Elings patent, specifies parabolic, spherical, or ellipsoidal mirrors. The existing state of technology in 1972 would have made aspheric mirrors an impractical consideration. Thus, the device described in the Elings patent could have only functioned acceptably using two parabolic mirrors. Today""s manufacturing technology, however, allows the production of aspheric optics in volume, and together with currently available desktop lens design software, makes the design and production of such complex optics possible. Parabolas are excellent for imaging at the focal point, but as one attempts to image larger objects where portions of the object are located substantially offset from the focal point, the effects of optical aberrations seriously degrade image quality. The aberrations and image degradation created by two spheres would have made the image nearly unrecognizable as the object being imaged. Ellipses have even more significant imaging problems. The parabola was the optimum solution in 1972, since production of aspheric optics of any size was not a practical option or something that one skilled in the art would even consider designing or building. Recent technological advances in lens manufacturing now make aspheric reflectors a practical solution to a difficult imaging problem.
An asphere is an optimized curve, significantly deviating from the other conic family of curves, such as spheres, parabolas, hyperbolas, and ellipses. Aspheres have very non-uniform curve changes that are specifically designed to counteract and minimize the aberrations that are natural phenomena of other curve families, especially for imaging off-axis or offset from the focal point.
U.S. Pat. No. 4,802,750, to Welck (1989), the complete disclosure of which is hereby incorporated herein by reference, referred to hereinafter as the Welck patent, discloses two facing parabolic segments of equal focal length, each being positioned such that its vertex is coincident with the focal point of the other. The light-path is transmitted from the focal point of the first parabolic mirror segment and is reflected off of the first parabolic reflector surface as collimated light (i.e., the reflected rays emanating from any one point source are substantially parallel to all other reflected rays emanating from the same source point, regardless of where on the curved surface it reflects from) as it is reflected to the second facing parabolic mirror, forming an image at the focal point of the second parabolic mirror. Maintaining a collimated or parallel light path between the two reflector surfaces is important to minimize the effects of aberrations, which is a natural phenomenon of curved optics, such as parabolic mirrors. The present invention differs substantially, in that the system of the Welck patent is limited to equal focal length parabolic segments, and is defined as an off-axis system. The Welck patent differs from the Elings patent, in that it uses xe2x80x9ccompound curvilinear surfaces of revolutionxe2x80x9d. Although the mirrors disclosed in the Welck patent are defined as having a xe2x80x9ccompound curvilinearxe2x80x9d surface of revolution, the Welck patent is clearly limited to parabolic surfaces.
In a conventional configuration, such as the Welck patent, using two parabolic mirrors of equal focal lengths, the light-path between the two parabolic reflectors is collimated when the image is projected at a xe2x80x9cone-to-onexe2x80x9d unmagnified condition. To create a de-magnified image using this configuration, the actual target object must be moved to a position other than the focal point. The result of de-magnifying with this method is that the light-path between the two parabolic mirrors is no longer collimated or parallel, and the effects of aberrations become more apparent, thus causing degradation of the projected image. As the image moves away from the focal point, the image quality degrades substantially. This is a natural and inherent problem with parabolic systems used off-axis, or when imaging at a point other than the focal point of the optical elements. An aspheric curve can be optimized to counteract and minimize such aberrations.
There are significant advantages to projecting a de-magnified image with improved imagery. A de-magnified image has a higher resolution per square inch. As an example, a standard 5xe2x80x3 LCD panel measuring 3xe2x80x3 high by 4xe2x80x3 wide, with 640 by 480 resolution has a resolution of 160 pixels per inch in both the horizontal and vertical direction, or 25,600 pixels per square inch. A real image projected by the present invention, using two unequal focal length mirror segments (e.g., one at 80% of the other, or an 80% de-magnification), at least one of which is aspheric in shape, results in a real image pixel density of 200 pixels per inch in both horizontal and vertical direction, thus resulting in an image pixel density of 40,000 pixels per square inch. Thus, the resulting resolution of the image is 156% of the resolution of the actual target LCD screen. The density of a real image relates directly to how solid and, thus, how real the image appears to the eye. This is of significance in preventing image xe2x80x9cbleed-throughxe2x80x9d of the background scene or image.
A second benefit of the present invention is that it increases the brightness per square inch of the projected real image, as compared to the actual target object, with significantly less image degradation. As an example, the system using a LCD panel that produces 200 lumens per square inch produces an image that provides 230 lumens per square inch (assuming that the two reflectors each have a reflectivity of 96% and the system has two different focal lengths, one being 80% of the other). In contrast, prior art systems, such as those described in the Elings and Welck patents, produce a real image having a brightness of only 184 lumens per square inch (assuming that the mirrors also have 96% reflective coatings and the systems are used in a 1xc3x97 magnification or equal focal lengths reflectors, as they are described).
An additional benefit of the present invention is that the optical orientation of the two aspheric mirrors optionally can be reversed, so that the axis of the longer focal length segment is parallel to the viewing axis, thus producing a magnified image at an increased projection distance. The two different focal length mirrors optionally are combined in four different orientations. For example, in a system using a 10xe2x80x3 focal length mirror and a 12xe2x80x3 focal length mirror, four separate effects can be achieved through varying the combination of focal lengths. Two 10xe2x80x3 mirrors would produce a 1xc3x97 full size image with increased field of view. Two 12xe2x80x3 mirrors would produce a 1xc3x97 full size image with greater projection distance. A 12xe2x80x3 primary mirror and 10xe2x80x3 secondary mirror would produce a de-magnified image, and a 10xe2x80x3 primary mirror and 12xe2x80x3 secondary mirror would produce a magnified image.
The most important advantage of an asphere over a parabola is that the optic is no longer limited by the 2.828 ratio of diameter to focal length. In a parabola, light emanating from the focal point will always reflect in a collimated beam, or parallel off the surface of the parabola up to a physical distance limit of (2.828xe2x80x3/2*f) from the vertex. For a 10xe2x80x3 focal length parabolic mirror, the maximum diameter that would reflect collimated light is a 28.28xe2x80x3 diameter optic. Light striking the parabolic surface outside of this physical diameter is not reflected in a collimated beam. Therefore, a parabola with a focal length of ten inches (10xe2x80x3) is limited to a diameter of 28.28xe2x80x3 or 2.828 times 10xe2x80x3. A parabola with a diameter larger than 2.828 times the focal length will form a distorted image. An aspheric curve is not limited by the 2.828 times the focal length factor. An asphere can be designed with a 10xe2x80x3 focal length that is larger than 28.28xe2x80x3 in diameter, and which will maintain a collimated reflected beam across the entire surface. If the aspheric curve is formed as a holographic mirror, the advantages of larger aspheric optics become apparent, especially for the xe2x80x9c360xe2x80x9d configuration.
In studying the ray-tracings of the various curves, it becomes apparent that using aspheric optics can significantly improve image quality, when the system is used in an off-axis arrangement. Thus, the configuration shown in the Welck patent, for example, could be substantially improved, by substituting the aspheric mirror of the present invention for one or both of the parabolic mirror segments described in the Welck patent. This same advantage or improvement also can be applied to the configuration shown in the Elings patent, thus providing a greatly improved image in the 360xc2x0 device described therein.
Briefly stated, a real image projection system includes at least two optical surfaces of the conical family of curves, wherein at least one of said optical curves comprises an aspherical surface of revolution. The system optionally includes a combination of any focal length curvatures, optionally comprising two curved optics or one curved optic comprising two optical surfaces of revolution, one on the convex and one on the concave side, one of which surfaces of revolution is an asphere.
In an embodiment of the invention, a real image projection system includes a pair of curved reflector segments of the conical family of curves, wherein at least one of the reflector segments has an aspherical surface of revolution, the primary segment being of longer focal length relative to the secondary segment, and an object positioned substantially at the focal point of the longer focal length reflector segment, such that a real image is positioned substantially at the focal point of the shorter focal length reflector segment, and the real image is projected along a viewing axis extending from the object positioned at the focal point of the primary reflector segment to the surface of the primary reflector segment, to the surface of the secondary reflector segment, to the focal point of the secondary reflector segment, to a viewer.
In a second embodiment of the invention, one optical element consists of two separate curves, one aspheric curve on the concave side of the optic, and one curve of the standard conic curve family on the convex side of the optic. A parabolic, spherical, or other standard conic curvature is provided on the convex surface, which has a mirrored coating applied, and the concave surface has an aspheric curvature and an anti-reflective coating, which functions as a corrective lens to reduce spherical aberration and other naturally occurring optical aberrations.